Plate Heat Exchangers
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Purpose of the Equipment: Plate heat exchangers (PHE), often called plate-andframe heat exchangers, are used to change the temperature of a liquid, vapor or gas media.
A thin, corrugated plate is used to transfer the heat from the media on one side of the plate to the media on the other side. 3/4/2014
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The plate heat exchanger consists of a frame with end plates which squeeze the corrugated heat transfer plates. Figure 2
shows a plate pack of corrugated plates with portholes for the media to flow.
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Principles of How the Equipment Works:
Plate heat exchangers use the thin plates to keep two media of
different temperatures apart while allowing heat energy to flow between them through the plate. The heat energy
transfer across the plate acts to change the temperatures of the two media. The hotter one becomes cooler, and the
colder one becomes hotter.
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Gasketed Gasket ed plate p late heat ex exchangers: changers: Plate thickness vary from 0.5 and 3 mm. Gap between two plates 1.5 to 5 mm. Plate surface areas range from 0.03 to 1.5 m 2, with a plate width : length ratio from 2.0 to 3.0. The size of plate heat exchangers can vary from 0.03 m 2 to 1500 m2. The maximum flow-rate of fluid is limited to around 2500 m 3/h. Plates are available in a wide range of metals and alloys; including stainless steel aluminum and titanium. A variety of gasket materials is also used. 3/4/2014
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Ad A dvantages
Plate heat exchangers exchangers are easier to maintain.
Low approach temps can be used, as low as 1 °C, compared with 5 to 10 "C for shell and tube exchangers. exchangers.
Plate heat exchangers are more flexible, it is easy to add extra plates.
Plate heat exchangers exchangers are more suitable for highly viscous materials.
The temperature correction factor, F t , will normally be higher with plate heat exchangers, exchangers, as the flow is closer to true tr ue countercurrent flow. flow.
Fouling tends to be significantly significantly less in plate heat exchangers. exchangers.
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Disadvantages: A
plate is not a good shape to resist pressure and plate heat
exchangers are not suitable for pressures greater than about 30 bar.
The selection of a suitable gasket is critical.
The maximum operating temperature is limited to about 250°C, due to the performance of the available available gasket materials. materials.
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PLATE PLA TE H EA EAT T E X CHA NGE R D ESIG N
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PROCEDURE The design procedure is similar to that for shell and tube exchangers. 1. Calculate Calcul ate duty, the rate rate of heat transfer required. 2. If the specification is incomplete, incomplete, determine determine the unknown fluid temperature temperature or fluid flow-rate from a heat balance. 3.
Calcul Calculat ate e the log mean mean tempe tempera ratur ture e diff differ erenc ence, e, ∆TLM.
4. Determi Determine ne the log mean mean temper temperatur ature e correction correction factor factor,, F t . 5. Calculate Calculate the correct corrected ed mean mean temper temperatur ature e differe difference nce ∆TM = F t x ∆TLM. 6. Estimat Estimate e the over overall all heat heat transf transfer er coeffici coefficient. ent. 7. Calcul Calculat ate e the surfa surface ce area area requ requir ired. ed. 8. Determin Determine e the number number of plates plates requir required ed = total total surface surface area/a area/area rea of one plate. 3/4/2014
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PROCEDURE 9.
Deci Decide de the the flow flow arran arrange geme ment nt and and numb number er of of pass passes es..
10. Calculate Calculate the the film heat transf transfer er coefficie coefficients nts for for each stream. stream. 11. Calculate Calculate the overal overalll coefficien coefficient, t, allowing allowing for fouling fouling factors. factors. 12. Compare Compare
the
calcul calculate ated d
coefficient. If satisfactory, say
with —0%
the
assumed assumed
overall overall
to + 10% error, proceed.
If unsatisfactory return to step 8 and increase or decrease the number of plates. 13. Check the press pressure ure drop for each stream. stream. 3/4/2014
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FLOW FL OW ARRANGEMEN TS
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Estimation of the temperature correction factor: For PHE it is convenient to express the LMTD correction factor, F t, as a function of the NTU, and the flow arrangement (number of passes). The correction will normally be higher for a PHE than for a STE operating with the same temperatures. For rough sizing purposes, the factor can be taken as 0.95 for series flow. The number of transfer units is given by: NTU = (t 0-t )/ i ∆TLM where t i = stream inlet temperature, °C, t o = stream outlet temperature, °C, ∆TLM =
log mean temperature difference, °C.
Typically, the NTU will range from 0.5 to 4.0, and for most applications will lie between 2.0 to 3.0. 3/4/2014
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Heat Transfer Transfer Coefficient:
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The corrugations on the plates will increase the projected plate area, and reduce the effective gap between the plates.
The channel width equals the plate pitch minus the plate thickness.
There is no heat transfer across the end plates, so the number of effective plates will be the total number of plates less two.
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